In recent years, the potential of viral vectors or genetically engineered viruses for the treatment of a variety of human diseases has been a topic of intense study worldwide. Herpes simplex virus (HSV) is among the most promising platforms for these purposes because of its efficient entry and spread into a wide range of cell types and its ability to accommodate expression cassettes for multiple or very large foreign genes that can provide therapeutic functions.
Targeting of HSV infection to specific cells for the delivery of therapeutic products or lytic infection of cancer cells requires (i) elimination of the native ability of the virus to interact with its entry receptors, mainly nectin-1 and HVEM, and (ii) the availability of a mechanism to trigger the virus entry process in response to virus engagement of alternate receptors. The attachment and fusion steps of HSV infection are mediated primarily by components of the viral envelope, a membranous structure containing at least 10 glycoproteins (gB, gC, gD, gE, gG, gH, gI, gJ, gL, and gM) and four non-glycosylated integral membrane proteins (UL20, UL34, UL45, and UL49.5). Of the glycoproteins, gB, gD, gH, and gL are essential for wild type herpes viruses to infect their host cells, while the remainder are dispensable for viral attachment or internalization. Prior to HSV-1 entry, virions are adsorbed to the cell surface through binding of gC and gB, to exposed glycosaminoglycans on the cell membrane. The entry process is then initiated by the interaction of gD with one of its cognate receptors, such as herpesvirus entry mediator (HVEM) or nectin-1. Receptor binding results in a conformational change in gD triggering activation of gB and a fourth envelope glycoprotein, gH, as the effectors of fusion between the viral envelope and cell membranes.
The virus can also infect cells by moving transcellularly, (e.g., at the sites of gap junctions), a process referred to as lateral spread. The process of lateral spread to neighboring cells also involves the envelope proteins; however different proteins appear to be essential for each process. Thus, for example, while gE, and gI are not essential for primary infection at the cell surface, removal of either of these greatly inhibits lateral spread.
Based on this understanding of the HSV-1 cell attachment and entry process, gC and gD have been modified to eliminate recognition of their natural receptors (“detargeting”) and insert a targeting element to provide a novel interaction with specific receptors on the target cell (“retargeting”). Although these approaches have shown promising results in terms of ablation of virus entry through the natural receptors, the efficiency of retargeted entry has not been universally high, thus limiting the practical application of these vectors. In fact, there has been only one example in the literature of efficient HSV-1 retargeting (Menotti et al., J. Virol., 82(20), 10153-61 (2008); Menotti et al., PNAS USA, 106(22) 9039-44 (2009)), and some attempts to take advantage of this design (replacement of residues 61-218 of gD with a single-chain antibody [scFv] against HER-2) to target the EGF receptor (EGFR) using an EGFR-specific scFv, have been unsuccessful.
It is clear, therefore, that a methodology is needed to enhance retargeted virus entry and spread, as such can reduce the effective virus dose and thereby increase safety.